Showing papers on "Imaging phantom published in 2012"

Ultrashort echo time imaging using pointwise encoding time reduction with radial acquisition (PETRA).

Abstract: Sequences with ultrashort echo times enable new applications of MRI, including bone, tendon, ligament, and dental imaging. In this article, a sequence is presented that achieves the shortest possible encoding time for each k-space point, limited by pulse length, hardware switching times, and gradient performance of the scanner. In pointwise encoding time reduction with radial acquisition (PETRA), outer k-space is filled with radial half-projections, whereas the centre is measured single pointwise on a Cartesian trajectory. This hybrid sequence combines the features of single point imaging with radial projection imaging. No hardware changes are required. Using this method, 3D images with an isotropic resolution of 1 mm can be obtained in less than 3 minutes. The differences between PETRA and the ultrashort echo time (UTE) sequence are evaluated by simulation and phantom measurements. Advantages of pointwise encoding time reduction with radial acquisition are shown for tissue with a T(2) below 1 ms. The signal to noise ratio and Contrast-to-noise ratio (CNR) performance, as well as possible limitations of the approach, are investigated. In-vivo head, knee, ankle, and wrist examples are presented to prove the feasibility of the sequence. In summary, fast imaging with ultrashort echo time is enabled by PETRA and may help to establish new routine clinical applications of ultrashort echo time sequences.

Frequency split metal artifact reduction (FSMAR) in computed tomography

Abstract: Purpose : The problem of metal artifact reduction (MAR) is almost as old as the clinical use of computed tomography itself. When metal implants are present in the field of measurement, severe artifacts degrade the image quality and the diagnostic value of CTimages. Up to now, no generally accepted solution to this issue has been found. In this work, a method based on a new MAR concept is presented: frequency split metal artifact reduction (FSMAR). It ensures efficient reduction of metal artifacts at high image quality with enhanced preservation of details close to metal implants. Methods : FSMAR combines a raw data inpainting-based MAR method with an image-based frequency split approach. Many typical methods for metal artifact reduction are inpainting-based MAR methods and simply replace unreliable parts of the projection data, for example, by linear interpolation. Frequency split approaches were used in CT, for example, by combining two reconstruction methods in order to reduce cone-beam artifacts. FSMAR combines the high frequencies of an uncorrected image, where all available data were used for the reconstruction with the more reliable low frequencies of an image which was corrected with an inpainting-based MAR method. The algorithm is tested in combination with normalized metal artifact reduction (NMAR) and with a standard inpainting-based MAR approach. NMAR is a more sophisticated inpainting-based MAR method, which introduces less new artifacts which may result from interpolation errors. A quantitative evaluation was performed using the examples of a simulation of the XCAT phantom and a scan of a spine phantom. Further evaluation includes patients with different types of metal implants: hip prostheses, dental fillings, neurocoil, and spine fixation, which were scanned with a modern clinical dual source CT scanner. Results : FSMAR ensures sharp edges and a preservation of anatomical details which is in many cases better than after applying an inpainting-based MAR method only. In contrast to other MAR methods, FSMAR yields images without the usual blurring close to implants. Conclusions : FSMAR should be used together with NMAR, a combination which ensures an accurate correction of both high and low frequencies. The algorithm is computationally inexpensive compared to iterative methods and methods with complex inpainting schemes. No parameters were chosen manually; it is ready for an application in clinical routine.

Comb-Push Ultrasound Shear Elastography (CUSE): A Novel Method for Two-Dimensional Shear Elasticity Imaging of Soft Tissues

Abstract: Fast and accurate tissue elasticity imaging is essential in studying dynamic tissue mechanical properties. Various ultrasound shear elasticity imaging techniques have been developed in the last two decades. However, to reconstruct a full field-of-view 2-D shear elasticity map, multiple data acquisitions are typically required. In this paper, a novel shear elasticity imaging technique, comb-push ultrasound shear elastography (CUSE), is introduced in which only one rapid data acquisition (less than 35 ms) is needed to reconstruct a full field-of-view 2-D shear wave speed map (40 × 38 mm). Multiple unfocused ultrasound beams arranged in a comb pattern (comb-push) are used to generate shear waves. A directional filter is then applied upon the shear wave field to extract the left-to-right (LR) and right-to-left (RL) propagating shear waves. Local shear wave speed is recovered using a time-of-flight method based on both LR and RL waves. Finally, a 2-D shear wave speed map is reconstructed by combining the LR and RL speed maps. Smooth and accurate shear wave speed maps are reconstructed using the proposed CUSE method in two calibrated homogeneous phantoms with different moduli. Inclusion phantom experiments demonstrate that CUSE is capable of providing good contrast (contrast-to-noise ratio ≥25 dB) between the inclusion and background without artifacts and is insensitive to inclusion positions. Safety measurements demonstrate that all regulated parameters of the ultrasound output level used in CUSE sequence are well below the FDA limits for diagnostic ultrasound.

Simultaneous Reconstruction of Activity and Attenuation in Time-of-Flight PET

Abstract: In positron emission tomography (PET) and single photon emission tomography (SPECT), attenuation correction is necessary for quantitative reconstruction of the tracer distribution. Previously, several attempts have been made to estimate the attenuation coefficients from emission data only. These attempts had limited success, because the problem does not have a unique solution, and severe and persistent “cross-talk” between the estimated activity and attenuation distributions was observed. In this paper, we show that the availability of time-of-flight (TOF) information eliminates the cross-talk problem by destroying symmetries in the associated Fisher information matrix. We propose a maximum-a-posteriori reconstruction algorithm for jointly estimating the attenuation and activity distributions from TOF PET data. The performance of the algorithm is studied with 2-D simulations, and further illustrated with phantom experiments and with a patient scan. The estimated attenuation image is robust to noise, and does not suffer from the cross-talk that was observed in non-TOF PET. However, some constraining is still mandatory, because the TOF data determine the attenuation sinogram only up to a constant offset.

Accurate Model-Based Reconstruction Algorithm for Three-Dimensional Optoacoustic Tomography

Abstract: In many practical optoacoustic imaging implementations, dimensionality of the tomographic problem is commonly reduced into two dimensions or 1-D scanning geometries in order to simplify technical implementation, improve imaging speed or increase signal-to-noise ratio. However, this usually comes at a cost of significantly reduced quality of the tomographic data, out-of-plane image artifacts, and overall loss of image contrast and spatial resolution. Quantitative optoacoustic image reconstruction implies therefore collection of point 3-D (volumetric) data from as many locations around the object as possible. Here, we propose and validate an accurate model-based inversion algorithm for 3-D optoacoustic image reconstruction. Superior performance versus commonly-used backprojection inversion algorithms is showcased by numerical simulations and phantom experiments.

Super-resolution methods in MRI: can they improve the trade-off between resolution, signal-to-noise ratio, and acquisition time?

Abstract: Improving the resolution in magnetic resonance imaging comes at the cost of either lower signal-to-noise ratio, longer acquisition time or both. This study investigates whether so-called super-resolution reconstruction methods can increase the resolution in the slice selection direction and, as such, are a viable alternative to direct high-resolution acquisition in terms of the signal-to-noise ratio and acquisition time trade-offs. The performance of six super-resolution reconstruction methods and direct high-resolution acquisitions was compared with respect to these trade-offs. The methods are based on iterative back-projection, algebraic reconstruction, and regularized least squares. The algorithms were applied to low-resolution data sets within which the images were rotated relative to each other. Quantitative experiments involved a computational phantom and a physical phantom containing structures of known dimensions. To visually validate the quantitative evaluations, qualitative experiments were performed, in which images of three different subjects (a phantom, an ex vivo rat knee, and a postmortem mouse) were acquired with different magnetic resonance imaging scanners. The results show that super-resolution reconstruction can indeed improve the resolution, signal-to-noise ratio and acquisition time trade-offs compared with direct high-resolution acquisition.

What Is the Best Way to Contour Lung Tumors on PET Scans? Multiobserver Validation of a Gradient-Based Method Using a NSCLC Digital PET Phantom

Abstract: Purpose To evaluate the accuracy and consistency of a gradient-based positron emission tomography (PET) segmentation method, GRADIENT, compared with manual (MANUAL) and constant threshold (THRESHOLD) methods. Methods and Materials Contouring accuracy was evaluated with sphere phantoms and clinically realistic Monte Carlo PET phantoms of the thorax. The sphere phantoms were 10–37 mm in diameter and were acquired at five institutions emulating clinical conditions. One institution also acquired a sphere phantom with multiple source-to-background ratios of 2:1, 5:1, 10:1, 20:1, and 70:1. One observer segmented (contoured) each sphere with GRADIENT and THRESHOLD from 25% to 50% at 5% increments. Subsequently, seven physicians segmented 31 lesions (7–264 mL) from 25 digital thorax phantoms using GRADIENT, THRESHOLD, and MANUAL. Results For spheres p 20 mm ( p p p p value p value Conclusion GRADIENT was the most accurate and consistent technique for target volume contouring. GRADIENT was also the most robust for varying imaging conditions. GRADIENT has the potential to play an important role for tumor delineation in radiation therapy planning and response assessment.

Initial Results of Simultaneous PET/MRI Experiments with an MRI-Compatible Silicon Photomultiplier PET Scanner

Abstract: The most investigated semiconductor photosensor for MRIcompatible PET detectors is the avalanche photodiode (APD). However, the silicon photomultiplier (SiPM), also called the Geiger-mode APD, is gaining attention in the development of the next generation of PET/MRI systems because the SiPM has much better performance than the APD. We have developed an MRI-compatible PET system based on multichannel SiPM arrays to allow simultaneous PET/MRI. Methods: The SiPM PET scanner consists of 12 detector modules with a ring diameter of 13.6 cm and an axial extent of 3.2 cm. In each detector module, 4 multichannel SiPM arrays (with 4 · 4 channels arranged in a 2 · 2 array to yield 8 · 8 channels) were coupled with 20 · 18 Lu1.9Gd0.1SiO5:Ce crystals (each crystal is 1.5 · 1.5 · 7 mm) and mounted on a charge division network for multiplexing 64 signals into 4 position signals. Each detector module was enclosed in a shielding box to reduce interference between the PET and MRI scanners, and the temperature inside the box was monitored for correction of the temperature-dependent gain variation of the SiPM. The PET detector signal was routed to the outside of the MRI room and processed with a field programmable gate array–based data acquisition system. MRI compatibility tests and simultaneous PET/MRI acquisitions were performed inside a 3-T clinical MRI system with 4-cm loop receiver coils that were built into the SiPM PET scanner. Interference between the imaging systems was investigated, and phantom and mouse experiments were performed. Results: No radiofrequency interference on the PET signal or degradation in the energy spectrum and flood map was shown during simultaneous PET/MRI. The quality of the MRI scans acquired with and without the operating PET showed only slight degradation. The results of phantom and mouse experiments confirmed the feasibility of this system for simultaneous PET/MRI. Conclusion: Simultaneous PET/MRI was possible with a multichannel SiPM-based PET scanner, with no radiofrequency interference on PET signals or images and only slight degradation of the MRI scans.

MRI-Based Nonrigid Motion Correction in Simultaneous PET/MRI

Abstract: Respiratory and cardiac motion is the most serious limitation to whole-body PET, resulting in spatial resolution close to 1 cm. Furthermore, motion-induced inconsistencies in the attenuation measurements often lead to significant artifacts in the reconstructed images. Gating can remove motion artifacts at the cost of increased noise. This paper presents an approach to respiratory motion correction using simultaneous PET/MRI to demonstrate initial results in phantoms, rabbits, and nonhuman primates and discusses the prospects for clinical application. Methods: Studies with a deformable phantom, a free-breathing primate, and rabbits implanted with radioactive beads were performed with simultaneous PET/MRI. Motion fields were estimated from concurrently acquired tagged MR images using 2 B-spline nonrigid image registration methods and incorporated into a PET list-mode ordered-subsets expectation maximization algorithm. Using the measured motion fields to transform both the emission data and the attenuation data, we could use all the coincidence data to reconstruct any phase of the respiratory cycle. We compared the resulting SNR and the channelized Hotelling observer (CHO) detection signal-to-noise ratio (SNR) in the motion-corrected reconstruction with the results obtained from standard gating and uncorrected studies. Results: Motion correction virtually eliminated motion blur without reducing SNR, yielding images with SNR comparable to those obtained by gating with 5–8 times longer acquisitions in all studies. The CHO study in dynamic phantoms demonstrated a significant improvement (166%–276%) in lesion detection SNR with MRI-based motion correction as compared with gating (P

Contrast-to-Noise Ratio and Low-Contrast Object Resolution on Full- and Low-Dose MDCT: SAFIRE Versus Filtered Back Projection in a Low-Contrast Object Phantom and in the Liver

Abstract: OBJECTIVE. The purpose of this article is to evaluate the effect of sinogram-affirmed iterative reconstruction (SAFIRE) on contrast-to-noise ratio (CNR) compared with filtered back projection (FBP) and to determine whether SAFIRE improves low-contrast object detection or conspicuity in a low-contrast object phantom and in the liver on full- and low-dose examinations. SUBJECTS AND METHODS. A low-contrast object phantom was scanned at 100%, 70%, 50%, and 30% dose using a single-source made of a dual-source MDCT scanner, with the raw data reconstructed with SAFIRE and FBP. Unenhanced liver CT scans in 22 patients were performed using a dual-source MDCT. The raw data from both tubes (100% dose) were reconstructed using FBP, and data from one tube (50% dose) were reconstructed using both FBP and SAFIRE. CNR was measured in the phantom and in the liver. Noise, contrast, and CNR were compared using paired Student t tests. Six readers assessed sphere detection and conspicuity in the phantom and liver–inferior ven...

The need for application-based adaptation of deformable image registration.

Abstract: Purpose: To utilize a deformable phantom to objectively evaluate the accuracy of 11 different deformable image registration (DIR) algorithms. Methods: The phantom represents an axial plane of the pelvic anatomy. Urethane plastic serves as the bony anatomy and urethane rubber with three levels of Hounsfield units (HU) is used to represent fat and organs, including the prostate. A plastic insert is placed into the phantom to simulate bladder filling. Nonradiopaque markers reside on the phantom surface. Optical camera images of these markers are used to measure the positions and determine the deformation from the bladder insert. Eleven different DIR algorithms are applied to the full and empty-bladder computed tomography images of the phantom (fixed and moving volumes, respectively) to calculate the deformation. The algorithms include those fromMIM Software (MIM) and Velocity Medical Solutions (VEL) and nine different implementations from the deformable image registration and adaptive radiotherapy toolbox for Matlab. These algorithms warp one image to make it similar to another, but must utilize a method for regularization to avoid physically unrealistic deformation scenarios. The mean absolute difference (MAD) between the HUs at the marker locations on one image and the calculated location on the other serves as a metric to evaluate the balance between image similarity and regularization. To demonstrate the effect of regularization on registration accuracy, an additional beta version of MIM was created with a variable smoothness factor that controls the emphasis of the algorithm on regularization. The distance to agreement between the measured and calculated marker deformations is used to compare the overall spatial accuracy of the DIR algorithms. This overall spatial accuracy is also utilized to evaluate the phantom geometry and the ability of the phantom soft-tissue heterogeneity to represent patient data. To evaluate the ability of the DIR algorithms to accurately transfer anatomical contours, the rectum is delineated on both the fixed and moving images. A Dice similarity coefficient is then calculated between the contour on the fixed image and that transferred, via the calculated deformation, from the moving to the fixed image. Results: The phantom possesses sufficient soft-tissue heterogeneity to act as a proxy for patient data. Large discrepancies appear between the algorithms and the measured ground-truth deformation. VEL yields the smallest mean spatial error and a Dice coefficient of 0.90. MIM produces the lowest MAD value and the highest Dice coefficient of 0.96, but creates the largest spatial errors. Increasing theMIM smoothness factor above the default value improves the overall spatial accuracy, but the factor associated with the lowest mean error decreases the Dice coefficient to 0.85. Conclusions: Different applications of DIR require disparate balances between image similarity and regularization. A DIR algorithm that is optimized only for its ability to transfer anatomical contours will yield large deformation errors in homogeneous regions, which is problematic for dose mapping. For this reason, these algorithms must be tested for their overall spatial accuracy. The developed phantom is an objective tool for this purpose.

Projection X-Space Magnetic Particle Imaging

Abstract: Projection magnetic particle imaging (MPI) can improve imaging speed by over 100-fold over traditional 3-D MPI. In this work, we derive the 2-D x-space signal equation, 2-D image equation, and introduce the concept of signal fading and resolution loss for a projection MPI imager. We then describe the design and construction of an x-space projection MPI scanner with a field gradient of 2.35 T/m across a 10 cm magnet free bore. The system has an expected resolution of 3.5 × 8.0 mm using Resovist tracer, and an experimental resolution of 3.8 × 8.4 mm resolution. The system images 2.5 cm × 5.0 cm partial field-of views (FOVs) at 10 frames/s, and acquires a full field-of-view of 10 cm × 5.0 cm in 4 s. We conclude by imaging a resolution phantom, a complex “Cal” phantom, mice injected with Resovist tracer, and experimentally confirm the theoretically predicted x-space spatial resolution.

IDEAL spiral CSI for dynamic metabolic MR imaging of hyperpolarized [1-13C]pyruvate

Abstract: Metabolic imaging with hyperpolarized [1-(13)C]pyruvate offers the unique opportunity for a minimally invasive detection of cellular metabolism. Efficient and robust acquisition and reconstruction techniques are required for capturing the wealth of information present for the limited duration of the hyperpolarized state (~1 min). In this study, the Dixon/IDEAL type of water-fat separation is expanded toward spectroscopic imaging of [1-(13) C]pyruvate and its down-stream metabolites. For this purpose, the spectral-spatial encoding is based on single-shot spiral image encoding and echo-time shifting in between excitations for the chemical-shift encoding. In addition, also a free-induction decay spectrum is acquired and the obtained chemical-shift prior knowledge is efficiently used in the reconstruction. The spectral-spatial reconstruction problem is found to efficiently separate into a chemical-shift inversion followed by a spatial reconstruction. The method is successfully demonstrated for dynamic, multislice [1-(13)C]pyruvate metabolic MR imaging in phantom and in vivo rat experiments.

MRI-Derived 3-D-Printed Breast Phantom for Microwave Breast Imaging Validation

Abstract: We propose a 3-D-printed breast phantom for use in preclinical experimental microwave imaging studies. The phantom is derived from an MRI of a human subject; thus, it is anthropomorphic, and its interior is very similar to an actual distribution of fibroglandular tissues. Adipose tissue in the breast is represented by the solid plastic (printed) regions of the phantom, while fibro glandular tissue is represented by liquid-filled voids in the plastic. The liquid is chosen to provide a biologically relevant dielectric contrast with the printed plastic. Such a phantom enables validation of microwave imaging techniques. We describe the procedure for generating the 3-D-printed breast phantom and present the measured dielectric properties of the 3-D-printed plastic over the frequency range 0.5-3.5 GHz. We also provide an example of a suitable liquid for filling the fibroglandular voids in the plastic.

Motion Correction in Dual Gated Cardiac PET Using Mass-Preserving Image Registration

Abstract: Respiratory and cardiac motion leads to image degradation in positron emission tomography (PET) studies of the human heart. In this paper we present a novel approach to motion correction based on dual gating and mass-preserving hyperelastic image registration. Thereby, we account for intensity modulations caused by the highly nonrigid cardiac motion. This leads to accurate and realistic motion estimates which are quantitatively validated on software phantom data and carried over to clinically relevant data using a hardware phantom. For patient data, the proposed method is first evaluated in a high statistic (20 min scans) dual gating study of 21 patients. It is shown that the proposed approach properly corrects PET images for dual-cardiac as well as respiratory-motion. In a second study the list mode data of the same patients is cropped to a scan time reasonable for clinical practice (3 min). This low statistic study not only shows the clinical applicability of our method but also demonstrates its robustness against noise obtained by hyperelastic regularization.

Iterative image reconstruction for cerebral perfusion CT using a pre-contrast scan induced edge-preserving prior

Abstract: Cerebral perfusion x-ray computed tomography (PCT) imaging, which detects and characterizes the ischemic penumbra, and assesses blood–brain barrier permeability with acute stroke or chronic cerebrovascular diseases, has been developed extensively over the past decades. However, due to its sequential scan protocol, the associated radiation dose has raised significant concerns to patients. Therefore, in this study we developed an iterative image reconstruction algorithm based on the maximum a posterior (MAP) principle to yield a clinically acceptable cerebral PCT image with lower milliampere-seconds (mA s). To preserve the edges of the reconstructed image, an edge-preserving prior was designed using a normal-dose pre-contrast unenhanced scan. For simplicity, the present algorithm was termed as ‘MAP-ndiNLM’. Evaluations with the digital phantom and the simulated low-dose clinical brain PCT datasets clearly demonstrate that the MAP-ndiNLM method can achieve more significant gains than the existing FBP and MAP-Huber algorithms with better image noise reduction, low-contrast object detection and resolution preservation. More importantly, the MAP-ndiNLM method can yield more accurate kinetic enhanced details and diagnostic hemodynamic parameter maps than the MAP-Huber method.

Dynamic light scattering optical coherence tomography

Abstract: We introduce an integration of dynamic light scattering (DLS) and optical coherence tomography (OCT) for high-resolution 3D imaging of heterogeneous diffusion and flow. DLS analyzes fluctuations in light scattered by particles to measure diffusion or flow of the particles, and OCT uses coherence gating to collect light only scattered from a small volume for high-resolution structural imaging. Therefore, the integration of DLS and OCT enables high-resolution 3D imaging of diffusion and flow. We derived a theory under the assumption that static and moving particles are mixed within the OCT resolution volume and the moving particles can exhibit either diffusive or translational motion. Based on this theory, we developed a fitting algorithm to estimate dynamic parameters including the axial and transverse velocities and the diffusion coefficient. We validated DLS-OCT measurements of diffusion and flow through numerical simulations and phantom experiments. As an example application, we performed DLS-OCT imaging of the living animal brain, resulting in 3D maps of the absolute and axial velocities, the diffusion coefficient, and the coefficient of determination.

Variance analysis of x-ray CT sinograms in the presence of electronic noise background.

Abstract: Purpose: Low-dose x-ray computed tomography (CT) is clinically desired Accurate noise modeling is a fundamental issue for low-dose CT image reconstruction via statistics-based sinogram restoration or statistical iterative image reconstruction In this paper, the authors analyzed the statistical moments of low-dose CT data in the presence of electronic noise background Methods: The authors first studied the statistical moment properties of detected signals in CT transmission domain, where the noise of detected signals is considered as quanta fluctuation upon electronic noise background Then the authors derived, via the Taylor expansion, a new formula for the mean–variance relationship of the detected signals in CT sinogram domain, wherein the image formation becomes a linear operation between the sinogram data and the unknown image, rather than a nonlinear operation in the CT transmission domain To get insight into the derived new formula by experiments, an anthropomorphic torso phantom was scanned repeatedly by a commercial CT scanner at five different mAs levels from 100 down to 17 Results: The results demonstrated that the electronic noise background is significant when low-mAs (or low-dose) scan is performed Conclusions: The influence of the electronic noise background should be considered in low-dose CT imaging

Technical performance evaluation of a human brain PET/MRI system

Abstract: Technical performance evaluation of a human brain PET/MRI system. The magnetic field compatible positron emission tomography (PET) insert is based on avalanche photodiode (APD) arrays coupled with lutetium oxyorthosilicate (LSO) crystals and slip-fits into a slightly modified clinical 3-T MRI system. The mutual interference between the two imaging techniques was minimised by the careful design of the hardware to maintain the quality of the B 0 and B 1 field homogeneity. The signal-to-noise ratio (SNR) and the homogeneity of the MR images were minimally influenced by the presence of the PET. Measurements according to the Function Biomedical Informatics Research Network (FBIRN) protocol proved the combined system’s ability to perform functional MRI (fMRI). The performance of the PET insert was evaluated according to the National Electrical Manufacturers Association (NEMA) standard. The noise equivalent count rate (NEC) peaked at 30.7 × 103 counts/s at 7.3 kBq/mL. The point source sensitivity was greater than 7 %. The spatial resolution in the centre field of view was less than 3 mm. Patient data sets clearly revealed a noticeably good PET and MR image quality. PET and MRI phantom tests and first patient data exhibit the device’s potential for simultaneous multiparametric imaging. • Combination of PET and MRI is a new emerging imaging technology. • Evaluated brain PET/MRI enables uncompromised imaging performance. • PET/MRI aims to provide multiparametric imaging allowing acquisition of morphology and metabolism.

Fast compressed sensing-based CBCT reconstruction using Barzilai-Borwein formulation for application to on-line IGRT

Abstract: Purpose: Compressed sensing theory has enabled an accurate, low-dose cone-beam computed tomography (CBCT) reconstruction using a minimal number of noisy projections. However, the reconstruction time remains a significant challenge for practical implementation in the clinic. In this work, we propose a novel gradient projection algorithm, based on the Gradient-Projection-Barzilai-Borwein formulation (GP-BB), that handles the total variation (TV)-norm regularization-based least squares problem for the CBCT reconstruction in a highly efficient manner, with speed acceptable for routine use in the clinic. Methods: CBCT is reconstructed by minimizing an energy function consisting of a data fidelity term and a TV-norm regularization term. Both terms are simultaneously minimized by calculating the gradient projection of the energy function with the step size determined using an approximate Hessian calculation at each iteration, based on the Barzilai-Borwein formulation. To speed up the process, a multiresolution optimization is used. In addition, the entire algorithm was designed to run with a single graphics processing unit (GPU) card. To evaluate the performance, the Shepp-Logan numerical phantom, the CatPhan 600 physical phantom, and a clinically-treated head-and-neck patient were acquired from the TrueBeam system (Varian Medical Systems, Palo Alto, CA). For each scan, in total, 364 projections were acquired in a 200more » deg. rotation. The imager has 1024 x 768 pixels with 0.388 x 0.388-mm resolution. This was down-sampled to 512 x 384 pixels with 0.776 x 0.776-mm resolution for reconstruction. Evenly spaced angles were subsampled and used for varying the number of projections for the image reconstruction. To assess the performance of our GP-BB algorithm, we have implemented and compared with three compressed sensing-type algorithms, the two of which are popular and published (forward-backward splitting techniques), and the other one with a basic line-search technique. In addition, the conventional Feldkamp-Davis-Kress (FDK) reconstruction of the clinical patient data is compared as well. Results: In comparison with the other compressed sensing-type algorithms, our algorithm showed convergence in {<=}30 iterations whereas other published algorithms need at least 50 iterations in order to reconstruct the Shepp-Logan phantom image. With the CatPhan phantom, the GP-BB algorithm achieved a clinically-reasonable image with 40 projections in 12 iterations, in less than 12.6 s. This is at least an order of magnitude faster in reconstruction time compared with the most recent reports utilizing GPU technology given the same input projections. For the head-and-neck clinical scan, clinically-reasonable images were obtained from 120 projections in 34-78 s converging in 12-30 iterations. In this reconstruction range (i.e., 120 projections) the image quality is visually similar to or better than the conventional FDK reconstructed images using 364 projections. This represents a dose reduction of nearly 67% (120/364 projections) while maintaining a reasonable speed in clinical implementation. Conclusions: In this paper, we proposed a novel, fast, low-dose CBCT reconstruction algorithm using the Barzilai-Borwein step-size calculation. A clinically viable head-and-neck image can be obtained within {approx}34-78 s while simultaneously cutting the dose by approximately 67%. This makes our GP-BB algorithm potentially useful in an on-line image-guided radiation therapy (IGRT).« less

Ultrasound contrast plane wave imaging

Abstract: Background: Monitoring the accumulation of microbubbles within tissue vasculature with ultrasound allows both molecular and perfusion imaging. Unfortunately, conventional imaging with focused pulses can destroy a large fraction of the microbubbles it is trying to follow. Using coherent synthetic summation, ultrafast plane wave imaging could attain similar image quality, while reducing the peak acoustic pressure and bubble disruption. Method: In these experiments, microbubbles were flowed in a wall-less vessel phantom. Images were obtained on a programmable clinical scanner with a set of line-per-line focused pulses for conventional contrast imaging and with compounded plane wave transmission adapted for nonlinear imaging. Imaging was performed between 14 and 650 kPa peak negative pressure at 7.5 MHz. The disruption of the microbubbles was evaluated by comparing the microbubble intensity before and after acquisition of a set of 100 images at various pressures. Results: The acoustic intensity required to disrupt 50% of the microbubbles was 24 times higher with plane-wave imaging compared with conventional focused pulses. Although both imaging approaches yield similar resolution, at the same disruption level, plane-wave imaging showed better contrast. In particular, at similar disruption ratio (50% after 100 images), contrast-pulse sequencing (CPS) performed with plane waves displayed an improvement of 11 dB compared with conventional nonlinear imaging. Conclusion: In each resolution cell of the image, plane-wave imaging spread the spatial peak acoustic intensity over more pulses, reducing the peak pressure and, hence, preserving the microbubbles. This method could contribute to molecular imaging by allowing the continuous monitoring of the accumulation of microbubbles with improved contrast.

Development and evaluation of an improved quantitative 90Y bremsstrahlung SPECT method

Abstract: Purpose: Yttrium-90 (Y-90) is one of the most commonly used radionuclides in targeted radionuclide therapy (TRT). Since it decays with essentially no gamma photon emissions, surrogate radionuclides (e.g., In-111) or imaging agents (e.g., Tc-99m MAA) are typically used for treatment planning. It would, however, be useful to image Y-90 directly in order to confirm that the distributions measured with these other radionuclides or agents are the same as for the Y-90 labeled agents. As a result, there has been a great deal of interest in quantitative imaging of Y-90 bremsstrahlung photons using single photon emission computed tomography (SPECT) imaging. The continuous and broad energy distribution of bremsstrahlung photons, however, imposes substantial challenges on accurate quantification of the activity distribution. The aim of this work was to develop and evaluate an improved quantitative Y-90 bremsstrahlung SPECT reconstruction method appropriate for these imaging applications. Methods: Accurate modeling of image degrading factors such as object attenuation and scatter and the collimator-detector response is essential to obtain quantitatively accurate images. All of the image degrading factors are energy dependent. Thus, the authors separated the modeling of the bremsstrahlung photons into multiple categories and energy ranges. To improve the accuracy, the authors used a bremsstrahlung energy spectrum previously estimated from experimental measurements and incorporated a model of the distance between Y-90 decay location and bremsstrahlung emission location into the SIMIND code used to generate the response functions and kernels used in the model. This improved Monte Carlo bremsstrahlung simulation was validated by comparison to experimentally measured projection data of a Y-90 line source. The authors validated the accuracy of the forward projection model for photons in the various categories and energy ranges using the validated Monte Carlo (MC) simulation method. The forward projection model was incorporated into an iterative ordered subsets-expectation maximization (OS-EM) reconstruction code to allow for quantitative SPECT reconstruction. The resulting code was validated using both a physical phantom experiment with spherical objects in a warm background and a realistic anatomical phantom simulation. In the physical phantom study, the authors evaluated the method in terms of quantitative accuracy of activity estimates in the spheres; in the simulation study, the authors evaluated the accuracy and precision of activity estimates from various organs and compared them to results from a previously proposed method. Results: The authors demonstrated excellent agreement between the experimental measurement and Monte Carlo simulation. In the XCAT phantom simulation, the proposed method achieved much better accuracy in the modeling (error in photon counts was -1.1 %) compared to a previously proposed method (errors were more than 20 %); the quantitative accuracy of activity estimates was excellent for all organs (errors were from -1.6 % to 11.9 %) and comparable to previously published results for I-131 using the same collimator. Conclusions: The proposed Y-90 bremsstrahlung SPECT reconstruction method provided very accurate estimates of organ activities, with accuracies approaching those previously observed for I-131. The method may be useful in verifying organ doses for targeted radionuclide therapy using Y-90. (C) 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.3700174] (Less)

Potential of dual-energy subtraction for converting CT numbers to electron density based on a single linear relationship.

Abstract: Purpose: The conversion of the computed tomography(CT) number to electron density is one of the main processes that determine the accuracy of patient dose calculations in radiotherapy treatment planning However, the CT number and electron density of tissues cannot be generally interrelated via a simple one-to-one correspondence because the CT number depends on the effective atomic number as well as the electron density The purpose of this study is to present a simple conversion from the energy-subtracted CT number (ΔHU) by means of dual-energy CT (DECT) to the relative electron density (ρe) via a single linear relationship Methods: The ΔHU–ρe conversion method was demonstrated by performing analytical DECT image simulations that were intended to imitate a second-generation dual-source CT (DSCT) scanner with an additional tin filtration for the high-kV tube The ΔHU–ρecalibration line was obtained from the image simulation with a 33 cm-diameter electron density calibration phantom equipped with 16 inserts including polytetrafluoroethylene, polyvinyl chloride, and aluminum; the elemental compositions of these three inserts were quite different to those of body tissues The ΔHU–ρe conversion method was also applied to previously published experimental CT data, which were measured using two different CTscanners, to validate the clinical feasibility of the present approach In addition, the effect of object size on ρe-calibrated images was investigated by image simulations using a 25 cm-diameter virtual phantom for two different filtrations: with and without the tin filter for the high-kV tube Results: The simulated ΔHU–ρe plot exhibited a predictable linear relationship over a wide range of ρe from 000 (air) to 235 (aluminum) Resultant values of the coefficient of determination, slope, and intercept of the linear function fitted to the data were close to those of the ideal case The maximum difference between the ideal and simulated ρe values was −07% The satisfactory linearity of ΔHU–ρe was also confirmed from analyses of the experimental CT data In the experimental cases, the maximum difference between the nominal and simulated ρe values was found to be 25% after two outliers were excluded When compared with the case without the tin filter, the ΔHU–ρe conversion performed with the tin filter yielded a lower dose and more reliable ρe values that were less affected by the object-size variation Conclusions: The ΔHU–ρecalibration line with a simple one-to-one correspondence would facilitate the construction of a well-calibrated ρeimage from acquired dual-kV images, and currently, second generation DSCT may be a feasible modality for the clinical use of the ΔHU–ρe conversion method

Development and validation of a practical lower-dose-simulation tool for optimizing computed tomography scan protocols.

Abstract: Objective The objective of this study was to develop and validate a novel noise insertion method that can accurately simulate lower-dose images from existing standard-dose computed tomography (CT) data. Methods The noise insertion method incorporates the effects of the bowtie filter, automatic exposure control, and electronic noise. We validated this tool using both phantom and patient studies. The phantom study compared simulated lower-dose images with the actually acquired lower-dose images. The patient studies included 105 pediatric and 24 adult CT body examinations. Results The noise level in the simulated images was within 3.2% of the actual lower-dose images in phantom experiments. Noise power spectrum also demonstrated excellent agreement. For the patient examinations, a mean difference of noise level between 2.0% and 9.7% was observed for simulated dose levels between 75% and 30% of the original dose. Conclusions An accurate technique for simulating lower-dose CT images was developed and validated, which can be used to retrospectively optimize CT protocols.

Accelerated phase‐contrast cine MRI using k‐t SPARSE‐SENSE

Abstract: Abnormalities in vascular flow patterns frequently can provide useful insights into pathophysiology. For example, recent studies suggest that abnormal hepatic venous waveforms can be used to diagnose liver pathologies such as cirrhosis (1–3). Although ultrasound is typically used in these studies, phase-contrast (PC) cine MRI (4–9) is a promising modality for studying hemodynamics and can be performed in addition to conventional contrast-enhanced anatomic MR imaging. Also, unlike Doppler ultrasonography, which is sensitive to misalignment between the vessel axis and Doppler beam angle, PC cine MRI can acquire data in any orientation, regardless of the depth of the vessel or overlying bowel gas. A major disadvantage of PC cine MRI, however, is its low data acquisition efficiency, which may limit the achievable spatial and temporal resolutions within clinically acceptable breath-hold durations. In PC cine MRI, two data sets (e.g., phase reference and velocity-encoded) are acquired to subtract the background phase due to static magnetic field inhomogeneities and susceptibility. Typically, phase reference and velocity-encoded cine data sets are acquired in an interleaved fashion within the same heart beat and over multiple heart beats (7). As a result of this time-consuming acquisition approach, PC cine MRI studies are particularly in need of acceleration to achieve relatively high spatio-temporal resolution within clinically acceptable scan times. Previous studies have incorporated acceleration methods into PC cine MRI, including spiral (10), radial (11), balanced steady-state free precession (12), view sharing (13,14), parallel imaging (15–17), and other spatio-temporal undersampling approaches (18,19), with each method having advantages and disadvantages. Among these acceleration methods, the most widely used method for PC cine MRI is parallel imaging (20–22). Most clinical MR scanners equipped with coil arrays are capable of performing generalized autocalibrating partially parallel acquisitions (GRAPPA) (22) or sensitivity encoding (SENSE) (21) accelerations with acceleration rates (R) of 2–3, but the achievable spatio-temporal resolution within a clinically acceptable breath-hold duration is still limited. Compressed sensing (CS) techniques, relatively recently introduced into the field of MRI (23), represent an alternative method to accelerate PC cine MRI (24–26). PC cine MRI is a good candidate for CS, because the background is in a steady state of magnetization, the temporal variation in signal is limited to blood vessel regions, and the resulting image data are sparse after applying an appropriate transform. Previous CS studies for dynamic MRI (27–32) have proposed temporal principal component analysis (PCA) as the sparsifying transform. To date, only a few preliminary CS studies for PC cine MRI have been reported (24–26). The study by Yoon et al. (24) describes use of a nonconvex greedy algorithm for high acceleration in a simulated phantom. The study by Tao et al. (26) reports low acceleration in retrospectively undersampled carotid PC cine MRI data, using temporal fast Fourier transform (FFT) as the sparsifying transform. The study by Velikina et al. (25) describes high acceleration using second temporal difference as the sparsifying transform. These preliminary studies have been important developments, but they have not been validated in vivo. This study describes a method to accelerate PC cine MRI using a combination of k-t SPARSE (33) and SENSE (21) that exploits joint sparsity among all component coil datasets (k-t SPARSE-SENSE) (34). Although this approach can be used in all vascular systems, the motivation of our work was to study hepatic blood flow waveforms in liver diseases (1–3) using accelerated PC cine MRI. We describe an accelerated PC cine MRI pulse sequence with k-t SPARSE-SENSE and our results validating it against a reference PC cine MRI pulse sequence with GRAPPA in a flow phantom and human subjects.

Real-time prompt γ monitoring in spot-scanning proton therapy using imaging through a knife-edge-shaped slit.

Abstract: In this paper we report on Monte Carlo simulations to investigate real-time monitoring of the track depth profile in particle therapy by measuring prompt gamma ray emissions: a high sensitivity imaging system employing a knife-edge-shaped slit combined with a position-sensitive gamma detector was evaluated. Calculations to test this new concept were performed for a head-sized software phantom. Clear spatial correlation is shown between the distribution of gamma rays detected with energies above 1.5 MeV and the distribution of prompt gamma rays emitted from the phantom. The number of neutrons originating from nuclear reactions in the phantom that are detected at these high energies is small. Most importantly it is shown that under common therapy conditions enough data may be collected during one spot-step (of the order of 10 ms) to locate the distal dose edge with a 1σ accuracy of better than 1 mm. This indicates that simple slit cameras have high potential for accurate real-time particle therapy adjustment and may become a practical way to improve particle therapy accuracy.